Regarding MPG, even if driven the same exact way, a Mazda 3 manual would get quite a bit better gas mileage. The 3-4mpg difference under easy driving is there because of the MPG decrease by having auto. My GF has an auto Mazda 3 sedan, and she gets about 30mpg highway on average, and it's gone as far as 35mpg before highway.It gets about 23-25 pure city. My neighbor has a 5 speed manual and gets noticably better gas mileage then the auto naturally.
If you mash the pedal down in the Speed 3 the way I did the one time I borrowed her car, you are gonna get WAY worse mileage. I can't speak for the Speed 3 exculsively, but my Speed 6 will guzzle gas if you don't take it easy in the city. You have to shift early in the first three gears, and minimize boost.
I'm sure the same rule applies in the Speed 3. No way you can average close to 24mpg combined city highway in that car unless you are taking it easy.